Thermal spraying, also known as flame spraying, involves the melting or at least heat softening of a heat fusible material such as metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto. A thermal spray gun is used for the purpose of heating and propelling the particles. In one type of thermal spray gun, the heat fusible material is supplied to the gun in powder form. Such powders typically comprise small particles, e.g., between 100 mesh U.S. Standard screen size (149 microns) and about 2 microns. Heat for powder spraying generally is provided by a combustion flame or an arc-generated plasma flame. The carrier gas, which entrains and transports the powder, may be one of the combustion gases or an inert gas such as nitrogen, or it may be compressed air.
Improved coatings may be produced by spraying at high velocity. For example, plasma spraying has proven successful for high velocity in many respects but it can suffer from non-uniform heating and/or poor particle entrainment which must be effected by feeding powder laterally into the high velocity plasma stream.
High velocity oxygen-fuel (HVOF) types of powder spray guns recently became practical and are typified in U.S. Pat. Nos. 4,416,421 and 4,865,252. This type of gun has a combustion chamber with a high pressure combustion effluent directed through a nozzle or open channel. Powder is fed into the nozzle chamber to be heated and propelled by the combustion effluent. Methods of spraying various materials with high velocity oxygen-fuel guns are taught in U.S. Pat. Nos. 4,999,225 and 5,006,321.
Another type of thermal spraying is effected with a detonation gun in which pulses of fuel mixture and powder are injected into a chamber with a long barrel and detonated. Successive high velocity bursts of the heated powder are directed to a substrate. This system is complex, costly and requires an enclosure against the noise bursts.
Wear resistance is a common requirement for thermal sprayed coatings, and carbide powders are frequently used, for example tungsten carbide. British patent specification No. 867,455 typifies cobalt bonded tungsten carbide powder admixed with a sprayweld self-fluxing powder for producing coatings. Often such coatings are subsequently fused. Self-fluxing alloys are nickel, cobalt or iron based alloys with chromium and with small amounts of boron, silicon and carbon which serve as fluxing agents and hardeners. Examples of self-fluxing alloys are disclosed in the aforementioned British patent specification and U.S. Pat. Nos. 3,743,533 and 4,064,608. Iron base alloys with molybdenum, boron and silicon are disclosed in U.S. Pat. No. 4,822,415.
The cobalt-tungsten carbide itself is also sprayed neat, i.e. without the self-fluxing ingredient, best results being with high velocity, particularly plasma spray or a high velocity oxygen-fuel (HVOF) gun or a detonation gun. The granules of a powder typically are formed of subparticles of tungsten carbide and cobalt, spray dried, sintered or fused, the result being crushed and classified into a powder of proper size for thermal spraying.
Another carbide is chromium carbide that is utilized for higher temperature applications. This carbide may be sprayed without any metal binder, but it usually is clad or bonded with nickel or nickel alloy, such as nickel-chromium alloy, such as described in U.S. Pat. Nos. 3,150,938 and 4,606,948.
Tungsten carbide and chromium carbide have been combined together with nickel for the detonation process as taught in U.S. Pat. Nos. 3,071,489. In one aspect of this patent, the elemental ingredients are all mixed together, and then sintered and crushed into a powder. In another aspect, separate powders of tungsten carbide, chromium carbide and nickel are blended to form a powder mixture of the three ingredients. In this form there is a tendency for the carbide to lose carbon in the flame. The two carbides also have been combined together with cobalt (without nickel) in a powder formed by casting and crushing, or by sintering, as taught in U.S. Pat. No. 4,925,626. Cobalt does not have as high corrosion resistance as nickel.
The latter patent teaches a method for producing a coating material of WC-Co-Cr alloy for high velocity oxygen-fuel thermal spraying. A mixture is prepared of tungsten carbide, cobalt and chromium, the latter being in the form of chromium carbide. The mixture is alloyed by by spray drying followed by sintering and plasma densification.
U.S. Pat. No. 4,588,608 teaches a powder for the detonation process, in which the powder is a cast and crushed composition of tungsten carbides, chrominum and cobalt. Two proprietary coatings of this nature are LW-45 and LW-15 produced by Praxair, Inc., Danbury, Conn., by the detonation process. LW-45 nominally contains 8% cobalt 4% chromium and balance tungsten carbide. LW-15 nominally contains 84% tungsten, 8% cobalt, 3% chromium and 5% carbon. These coatings have been utilized in specified applications such as petrochemical gate valves.